Do I need to worry about mosquitoes, malaria and dengue in Nepal?

The short answer

Mosquito-borne illness is mostly a low-altitude matter. The hot, humid southern plains, including jungle areas such as Chitwan and Bardia, are where mosquitoes are most active, especially in and after the monsoon. Kathmandu, Pokhara and the high hills have fewer mosquitoes in the cooler months, and treks above the low valleys have very few.

Simple habits help everywhere: use a repellent you trust, wear long sleeves and trousers at dusk, sleep under a net or in a screened room when offered, and empty standing water around you. Ask your doctor whether any medicine is advised for your route and season, and see a doctor promptly if you get a fever during or after the trip.